Field of the Invention
The present invention relates to transition metal oxide-containing cerium dioxide particles. More specifically, the present invention relates to transition metal oxide-containing cerium dioxide particles (hereinafter simply referred to as “oxide-containing particles”) that exert a catalyst performance by which nitrogen oxide and the like can be purified without increasing the use amount of any precious metal catalyst.
Description of the Related Art
In recent years, plural filters for purification are disposed in the midstream of an exhaust gas pathway of an internal-combustion engine such as a diesel engine for the purpose of a purification treatment of exhaust gas emitted from said engine. For example, a diesel particulate filter (DPF) is used for a purification treatment of exhaust gas from a diesel engine. Furthermore, in accordance with further tightening of the regulations in the emission standard of exhaust gas, a selective catalytic reduction (SCR) catalyst converter using an SCR catalyst having a function to selectively reduce components to be purified in exhaust gas by a reduction reaction, and the like are installed in the downstream position of the above-mentioned DPF (see FIG. 4).
DPF has a function to trap particulate matters (PM) such as soot, which are mainly present in the exhaust gas, by a porous honeycomb structure to thereby prevent said particulate matters from being released as they are into the air. On the other hand, an SCR catalyst converter can reduce nitrogen oxides (NOx) in the exhaust gas into nitrogen gas and water by utilizing ammonia (NH3) generated by the decomposition of urea injected from a urea injector disposed at the upper stream side of an exhaust gas pathway.
In the above-mentioned DPF, a metal catalyst containing a precious metal is loaded by generally a honeycomb structure made of a ceramic so as to remove carbon oxide (CO) and hydrocarbon (HC) by oxidation. Therefore, it is necessary to carry out a combustion regeneration treatment for removing soot deposited in the honeycomb structure by purifying exhaust gas by means of combustion. At this time, the combustion of the soot is promoted by the loaded catalyst.
On the other hand, in a urea SCR system, it is known that it is preferable to set the ratio of nitrogen monoxide (NO) to nitrogen dioxide (NO2) that flow into an SCR catalyst converter to 1:1 so as to decompose nitrogen oxides into nitrogen gas and water, in view of reaction rate. However, in a DPF constituted by a diesel oxidation catalyst (DOC) and a catalyzed soot filter (CSF) (see FIG. 4) positioned upstream of an SCR catalyst converter, nitrogen dioxide (NO2) is consumed for combusting particulate matters (PM). Therefore, the amount of the nitrogen monoxide (NO) in the gas emitted from the DPF and introduced in the SCR catalyst converter is considerably larger than the amount of the nitrogen dioxide (NO2). Therefore, the above-mentioned preferable ratio cannot be maintained, and thus there is a possibility that the purification efficiency of nitrogen oxide (NOx) is decreased.
Therefore, an attempt to bring a ratio of nitrogen monoxide to nitrogen dioxide in gas to be introduced in an SCR catalyst converter closer to a preferable ratio of 1:1 is executed. At this time, in order to set the ratio of the nitrogen monoxide to the nitrogen dioxide to 1:1, a part of the nitrogen monoxide is converted to nitrogen dioxide by oxidization by loading a precious metal catalyst such as platinum on a CSF in a subsequent stage of a DPF. However, since a precious metal catalyst such as platinum is expensive, it is highly possible that the costs for the entirety of the DPF increase. In addition, there is also a problem that, since a precious metal catalyst has strong oxidation power, it is difficult to adjust the ratio of nitrogen monoxide to nitrogen dioxide to a suitable ratio of 1:1.
In addition, in patent applications relating to a whole exhaust gas purification system, systems using cobalt monoxide (CoO), manganese dioxide (MnO2) or zirconium monoxide (ZrO) as an oxidation catalyst at a preceding stage of an SCR catalyst converter are shown (see Patent Documents 1 to 3 and the like). Furthermore, a catalyst-carrying honeycomb using an oxide catalyst (see Patent Document 4) or an exhaust gas purification device having a catalyst for purifying nitrogen oxides (see Patent Document 5) has been already disclosed.
[Patent Document 1] JP-A-2004-100699
[Patent Document 2] JP-A-5-195756
[Patent Document 3] JP-A-2014-57951
[Patent Document 4] JP-A-2008-302355
[Patent Document 5] JP-A-2006-346605